Zirconia sintered body, zirconia composition, zirconia pre-sintered body and preparing method thereof, and dental prosthesis

ABSTRACT

A method for preparing a zirconia composition comprises preparing a plurality of powders for lamination; the lamination powders containing zirconia, a stabilizer(s) suppressing phase transition of zirconia and a pigment(s) at respective different pigment content ratios, and laminating the lamination powders in a mold. In the laminating step, the mold is vibrated after at least two lamination powders are charged into the mold.

CROSS-REFERENCE TO RELATED APPLICATION

The present application is based upon and claims the benefit of thepriority of Japanese patent application No. 2013-100619 filed on May 10,2013, the disclosure of which is incorporated herein in its entirety byreference thereto.

TECHNICAL FIELD

The present invention relates to a zirconia sintered body. The presentinvention also relates to a composition and pre-sintered body (calcinedbody) for manufacturing the zirconia sintered body. The presentinvention also relates to methods of preparing the zirconia sinteredbody, composition and pre-sintered body. The present invention furtherrelates to a dental prosthesis (prosthetic material) having the zirconiasintered body.

BACKGROUND

Ceramics made of sintered body of zirconium oxide (IV) (ZrO₂) (referredto as “zirconia” hereinafter) has been used in various fields. Thezirconia sintered body having high strength has been applied for adental prosthetic material, tool, etc., for example. In design of such azirconia product, change in colors is often required. The zirconiasintered body is used as artificial teeth that are a substitutionmaterial in dental treatment, for example. With the artificial teeth, anappearance similar to that of natural teeth is required.

Patent Literature 1 discloses a multi-colored shaped body having layersarranged on top of one another for manufacture of dental restorations.The shaped body disclosed in Patent Literature 1 has (a) at least twosuccessive and differently colored main layers, and (b) at least twodifferently colored intermediate layers between the at least twosuccessive and differently colored main layers, wherein change in colorbetween the intermediate layers takes place in a direction which isreverse to a direction of the change in color between the main layers.

CITATIONS LIST Patent Literature

PATENT LITERATURE 1: JP Patent Kokai Publication No. JP2008-68079A

SUMMARY Technical Problem

The entire contents of disclosure of the above mentioned PatentLiterature 1 are to be incorporated herein by reference. The followinganalysis is given from the perspective of the present invention.

As the shaped body disclosed in Patent Literature 1, in case wherelayers of different colors are merely laminated, entire change in colorappears in stages (like stairs). That is, smooth gradation (colorchanges like a slope) can not be obtained. In particular, in the shapedbody disclosed in Patent Literature 1, two intermediate layers arearranged between adjacent main layers. The direction of the change incolor between these two intermediate layers is reversed to that of theentire change in color. Therefore, according to the shaped bodydisclosed in Patent Literature 1, natural gradation can not be realized.

Further, in a method disclosed in Patent Literature 1, in a case wherethe product is made of four colored main layers, for example, at leasteight layers of the main layers and intermediate layers must belaminated. Therefore, the method disclosed in Patent Literature 1 needsmuch works and thus time costs.

Solution to the Problem

According to a first aspect of the present invention, 1. a method forpreparing a zirconia composition is provided, the method comprising:preparing a plurality of powders adapted to form a lamination of layersof the plurality of powders containing zirconia, a stabilizer(s)suppressing phase transition of zirconia and a pigment(s) at respectivedifferent pigment content ratios; and laminating layers of the pluralityof powders for lamination in a mold. In the laminating step, the mold isvibrated after at least two layers of the powders for lamination arecharged into the mold.

According to a second aspect of the present invention, a method forpreparing a zirconia composition is provided, the method comprising;preparing a low addition ratio powder and a high addition ratio powdereach containing zirconia, a stabilizer(s) suppressing phase transitionof zirconia and a pigment(s), the low addition ratio powder and the highaddition ratio powder differing in pigment content ratios from oneanother; mixing the low addition ratio powder and the high additionratio powder to form at least one lamination powder; and laminating atleast two out of the low addition ratio powder, high addition ratiopowder and the lamination powder into the mold.

According to a third aspect of the present invention, a method forpreparing a zirconia pre-sintered body (may be termed “calcined body”)is provided, the method comprising the method for preparing the zirconiacomposition according to the present invention, and firing thecomposition at 800° C. to 1200° C.

According to a fourth aspect of the present invention, a method forpreparing a zirconia sintered body is provided, the method comprisingthe method for preparing the zirconia composition according to thepresent invention, and firing the composition at 1400° C. to 1600° C.

According to a fifth aspect of the present invention, a method forpreparing a zirconia sintered body is provided, the method comprisingthe method for preparing the zirconia pre-sintered body according to thepresent invention, and firing the pre-sintered body at 1400° C. to 1600°C.

According to a sixth aspect of the present invention, a zirconiasintered body is provided, which is manufactured by the method accordingthe present invention.

According to a seventh aspect of the present invention, a zirconiapre-sintered body is provided, which is manufactured by the methodaccording the present invention.

According to an eighth aspect of the present invention, a zirconiacomposition is provided, which is manufactured by the method accordingthe present invention

According to a ninth aspect of the present invention, a dentalprosthesis is provided, which is obtained by milling, grinding and/orcutting the pre-sintered body according to the present invention andsubsequently sintering the pre-sintered body.

Advantageous Effects of the Invention

The present invention has at least one of the following advantageouseffects.

According to the present invention, a zirconia sintered body havingnatural gradation can be obtained.

The zirconia sintered body, described above, can be obtained from thecomposition as well as the pre-sintered body according to the presentinvention.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic view for illustrating a three-point bending testmethod.

FIG. 2 is a schematic view of a zirconia sintered body.

FIG. 3 is a schematic view showing a test sample used for measuringdeformation at the time of sintering.

FIG. 4 is a schematic view showing a test sample used for measuringdeformation at the time of sintering.

FIG. 5 is a schematic view for illustrating a method for measuringdeformation.

FIG. 6 is a schematic view of a test sample used in Example 5, with agraph showing the results measured.

FIG. 7 is a schematic view of a test sample used in Comparative Example3, with a graph showing the results measured.

MODES

Preferred modes of the above respective aspects are now shown.

According to a preferred mode of the first aspect, in the laminatingstep, the mold is vibrated each time one powder for lamination ischarged into the mold.

According to a preferred mode of the second aspect, in the laminatingstep, the mold is vibrated after charging at least two powders into themold.

According to a preferred mode of the second aspect, in the mixing step,two or more powders for lamination with different mixing ratios of thelow addition ratio powder and the high addition ratio powder areprepared. In the laminating step, the powders are laminated one onanother so that the contents of the low addition ratio powder and thehigh addition ratio powder are varied in order.

According to a preferred mode of the first and second aspects, in thelaminating step, after charging one powder into the mold, an uppersurface of the powder is made flat.

According to a preferred mode of the first and second aspects, in thelaminating step, the powders are laminated so that the pigment contentsin the powders are varied in order.

According to a preferred mode of the sixth aspect, when the zirconiasintered body is prepared by sintering the zirconia composition, aflexural strength of the zirconia sintered body pursuant to JISR1601 isnot less than 1100 MPa as measured with a load point of a three-pointbending test aligned with a position of an interlayer boundary oflamination of the zirconia powders, the interlayer boundary traversingthe test sample of the zirconia sintered body along a direction of loadapplication.

According to a preferred mode of the sixth aspect, the flexural strengthis not less than 1200 MPa.

According to a preferred mode of the sixth aspect, when a zirconiapre-sintered body is prepared by pre-sintering the zirconia compositionat 800° C. to 1200° C., the flexural strength of the pre-sintered bodyas measured with a load point of a three-point bending test aligned withthe interlayer boundary pursuant to JISR1601 is not less than 90% of theflexural strength of the zirconia pre-sintered body obtained onpre-sintering one composition of the zirconia powders alone at the sametemperature as a pre-sintering temperature of the test sample, theinterlayer boundary traversing the test sample of the pre-sintered bodyalong a direction of load application.

According to a preferred mode of the sixth aspect, when the compositionis pre-sintered at 800° C. to 1200° C. to form a zirconia pre-sinteredbody, the pre-sintered body is shaped to a form of a rectangularparallelepiped 50 mm in width[length], 10 mm in height and 5 mm indepth[thickness] as a test sample, and two surfaces of the test sampleof 50 mm in width and 5 mm in depth are taken to be bottom surfaces;boundary surfaces formed by lamination of the zirconia powders thenextending in the same direction as the bottom surfaces, the test sampleis fired at 1500° C. for two hours, and the test sample is placed on aground with one of the two bottom surfaces that has been deformed to aconcave shape directed downwards, (a maximum gap between the deformedconcave bottom surface and a ground surface)/(distance between portionsof the test sample contacting the ground surface along the widthwisedirection)×100 is 0.15 or less.

According to a preferred mode of the sixth aspect, it is assumed that,on a straight line extending in a first direction from one end to anopposite end, a chromaticity (L*, a*, b*) in an L*a*b* colorchromaticity diagram at a first point in a domain from one end to up to25% of a total length of the straight line is (L1, a1, b1), and achromaticity (L*, a*, b*) in the L*a*b* color chromaticity diagram at asecond point in a domain from the opposite end to up to 25% of the totallength of the straight line is (L2, a2, b2). Then, L1 is not less than58.0 and not larger than 76.0, a1 is not less than −1.6 and not largerthan 7.6, b1 is not less than 5.5 and not larger than 26.7, L2 is notless than 71.8 and not larger than 84.2, a2 is not less than −2.1 andnot larger than 1.8, b2 is not less than 1.9 and not larger than 16.0,L1<L2, a1>a2, b1>b2, increasing or decreasing tendency of thechromaticity in the L*a*b* color chromaticity diagram not being changed.

According to a preferred mode of the sixth aspect, there is no domain onthe straight line interconnecting the first and second points where theL* value decreases by not less than unity (one) from a first pointtowards a second point. Also, there is no domain where the value of a*increases by not less than unity from the first point towards the secondpoint, while there is no domain where the value of b* increases by notless than unity from the first point towards the second point.

According to a preferred mode of the sixth aspect, it is assumed that,on the straight line interconnecting the first and second points, thechromaticity (L*, a*, b*) in the L*a*b* color chromaticity diagram at athird point intermediate between the first and second points is (L3, a3,b3). Then, L3 is not less than 62.5 and not larger than 80.5, a3 is notless than −1.8 and not larger than 5.5, b3 is not less than 4.8 and notlarger than 21.8, L1<L3<L2, a1>a3>a2 and b1>b3>b2.

According to a preferred mode of the sixth aspect, it is assumed that,on the straight line interconnecting the first and second points, thechromaticity (L*, a*, b*) in the L*a*b* color chromaticity diagram at afourth point intermediate between the third and second points is (L4,a4, b4). Then, L4 is not less than 69.1 and not larger than 82.3, a4 isnot less than −2.1 and not larger than 1.8, b4 is not less than 3.5 andnot larger than 16.2, L1<L3<L4<L2, a1>a3>a4>a2 and b1>b3>b4>b2.

According to a preferred mode of the sixth aspect, the third point is ata distance from the one end equal to 45% of the total length, the fourthpoint is at a distance from the one end equal to 55% of the totallength.

According to a preferred mode of the sixth aspect, the differencebetween the L* values of two neighboring ones of a first point, a thirdpoint, a fourth point and a second point is ΔL*, the difference betweenthe values of a* of two neighboring points is Δa*, the differencebetween the values of b* of two neighboring points is Δb* and the ΔE*abis calculated from the equation 1 shown below. Then, ΔE*ab between thefirst and third points is not less than 3.7 and not larger than 14.3,ΔE*ab between the third and fourth points is not less than 1.8 and notlarger than 10.5 and ΔE*ab between the fourth and second points is notless than 1.0 and not larger than 9.0

ΔE*ab=√{square root over ((ΔL*)²+(Δa*)²+(Δb*)²)}{square root over((ΔL*)²+(Δa*)²+(Δb*)²)}{square root over((ΔL*)²+(Δa*)²+(Δb*)²)}  [Equation 1]

According to a preferred mode of the sixth aspect, it is assumed that,on the straight line interconnecting the first and second points, thechromaticity (L*a*b*) in the L*a*b* color chromaticity diagram of thethird point located intermediate between the first and second points is(L3, a3, b3). Then L3 is not less than 69.1 and not larger than 82.3, a3is not less than −2.1 and not larger than 1.8, b3 is not less than 3.5and not larger than 16.2, L1<L3<L2, a1>a3>a2 and b1>b3>b2.

According to a preferred mode of the sixth aspect, the color is changedin the first direction extending from one end to the opposite end. Onthe straight line extending from the one end to the opposite end,increasing or decreasing tendency for the chromaticity in the L*a*b*color chromaticity diagram is not changed.

According to a preferred mode of the sixth aspect, on the straight lineinterconnecting the one end and the opposite end, the L* value tends toincrease, while the a* value as well as the b* value tends to decrease,from the first point towards the second point.

According to a preferred mode of the sixth aspect, the distance from theone end to the opposite end is 5 mm to 18 mm.

According to a preferred mode of the sixth aspect, there is no colorchange along a second direction perpendicular to the first direction.

According to a preferred mode of the sixth aspect, it is assumed that,at two points on a straight line extending in the second direction, thedifference between the L* values at two points is ΔL*, the differencebetween the a* values at the two points is Δa*, the difference betweenthe b* values at the two points is Δb* and ΔE*ab is calculated from theequation 1. Then, E*ab is less than unity.

According to a preferred mode of the sixth aspect, flexural strength asmeasured pursuant to JISR1601 is not less than 1000 MPa.

According to a preferred mode of the sixth aspect, fracture toughness asmeasured pursuant to JISR1607 is not less than 3.5 MPa·m^(1/2).

According to a preferred mode of the sixth aspect, in an X-raydiffraction pattern of a zirconia sintered body following a hydrothermaltreatment test at 180° C. and 1 MPa for five hours, a ratio of a heightof a peak existing in the vicinity of a [11-1] peak ascribable to amonoclinic crystal in the vicinity of 28° of 2θ to a height of a peakexisting in the vicinity of a [111] peak ascribable to a tetragonalcrystal in the vicinity of 30° of 2θ not larger than unity.

According to a preferred mode of the seventh aspect, a flexural strengthof a test sample of the zirconia pre-sintered body, measured pursuant toJISR1601, is not less than 90% of a flexural strength of a comparativezirconia pre-sintered body; the comparative zirconia pre-sintered bodybeing formed by pre-sintering one of the zirconia powders alone at thesame temperature as a pre-sintering temperature of the test sample; theflexural strength being measured under a condition that a load point ofa three-point bending test is positioned at a position of an interlayerboundary of the zirconia powders, the interlayer boundary traversing thetest sample of the sintered body along a direction of load application.

According to a preferred mode of the seventh aspect, when thepre-sintered body is shaped to a form of a rectangular parallelepiped 50mm in width[length], 10 mm in height and 5 mm in depth[thickness] as atest sample, two surfaces of the test sample of 50 mm in width and 5 mmin depth, are taken to be bottom surfaces; boundary surfaces formed bylamination of the zirconia powders extending in the same direction asthe bottom surfaces; the test sample is fired at 1500° C. for two hours;and the test sample is placed on a ground with one of the two bottomsurfaces that has been deformed to a concave shape directed downwards;(a maximum gap between the deformed concave bottom surface and a groundsurface)/(distance between portions of the test sample contacting theground surface along a widthwise direction)×100 is 0.15 or less.

According to a preferred mode of the ninth aspect, milling, grindingand/or cutting is performed by a CAD/CAM system.

According to the present invention, if the zirconia sintered body has ashape of a crown, preferably the ‘one end’ and the ‘opposite end’ denoteone point in an end on an incisal side and one point in an end on a rootside. The one point may be a point on an end face or on across-sectional face. The point located in a domain within 25% of thetotal length from the one end or the opposite end denotes a point thatspans a distance equivalent to 10% of a crown height apart from the oneend or the opposite end.

In case where the zirconia sintered body has a shape of a disc or ahexahedron such as a rectangular parallelepiped, the ‘one end’ or the‘opposite end’ preferably denotes a point on the upper surface or thelower surface (bottom surface). The one point may be a point on an endface or on a cross-sectional face. The point located in a domain fromone end or the opposite end to a point corresponding to 25% of the totallength denotes a point that spans a distance equivalent to 10% of thethickness of the disc or the hexahedron apart from the one end or theopposite end.

According to the present invention, the ‘first direction extending fromone end to the opposite end’ denotes a direction along which the colorchanges. As an example, the first direction is preferably the directionof laminating powders in a fabrication method as later explained. If,for example, the zirconia sintered body has the shape of a crown, thefirst direction is preferably a direction interconnecting the incisalside and the root side.

The zirconia sintered body of the present invention will now beexplained. The zirconia sintered body of the present invention is mainlycomposed of partially stabilized zirconia crystal grains sinteredtogether, and includes partially stabilized zirconia as a matrix phase.In the zirconia sintered body of the present invention, the principalcrystal phase of zirconia is tetragonal crystal or tetragonal crystalplus cubical crystal. Preferably, the zirconia sintered body issubstantially free of the monoclinic crystal in the state prior totreatment with hydrothermal testing as later explained.

The zirconia sintered body encompasses not only one obtained onsintering the shaped zirconia particles together at normal pressure orunder a non-pressurizing state but also that obtained by subjecting thesintered body to high temperature compression such as hot isostaticpressing (HIP) for compacting and densification.

The zirconia sintered body according to the present invention containszirconia and its stabilizer(s). The stabilizer(s) suppresses phasetransition of the zirconia of the tetragonal system to the monoclinicsystem. By suppressing the phase transition, it is possible to elevatestrength, durability as well as dimensional stability. As thestabilizer(s), oxides such as calcium oxide (CaO), magnesium oxide(MgO), yttrium oxide (Y₂O₃), referred to below as ‘yttria’, and ceriumoxide (CeO₂) may be given, for example. Preferably, such an amount ofthe stabilizer(s) that will cause zirconia particles of the tetragonalsystem to be partially stabilized is added. For example, if yttria isused as the stabilizer, the content of yttria is preferably 2.5 mol % to5 mol %, more preferably 3 mol % to 4.5 mol % and further preferably 3.5mol % to 4.5 mol % relative to the total of mols of zirconia and yttriasummed together. If the content of the stabilizer(s) is too high, theflexural strength as well as the fracture toughness is lowered, eventhough the phase transition is suppressed. If conversely the content ofthe stabilizer is too low, suppression of the progress of phasetransition is insufficient even though the deterioration of the flexuralstrength as well as fracture toughness could be suppressed. By the way,zirconia of the tetragonal system, partially stabilized by addition ofthe stabilizer, is termed partially stabilized zirconia (PSZ).

Preferably, the zirconia sintered body of the present invention containsaluminum oxide Al₂O₃ (alumina). Preferably, addition of aluminum oxidemay improve strength. The content of aluminum oxide in the zirconiasintered body is preferably 0 mass % (no aluminum oxide content) to 0.3mass % relative to the total mass of zirconia and the stabilizer. If thealuminum oxide content exceeds 0.3 mass %, the sintered body isdeteriorated in transparency. [Translator's Note: “mass %” issubstantially equivalent to “weight %”.]

Preferably, the zirconia sintered body of the present invention containstitanium oxide TiO₂ (titania). The content of titanium oxide may promotegrain growth. The content of titanium oxide in the zirconia sinteredbody is preferably 0 mass % (no titanium oxide) to 0.6 mass % relativeto the total mass of zirconia and the stabilizer. If the titanium oxidecontent exceeds 0.6 mass %, strength is deteriorated.

In the zirconia sintered body of the present invention, the content ofsilicon oxide SiO₂ (silica) is preferably not larger than 0.1 mass %relative to the total mass of zirconia and the stabilizer. The zirconiasintered body preferably substantially contains no silicon oxide. Thereason is that, if silicon oxide is contained, the zirconia sinteredbody is deteriorated in transparency. By the phrase ‘substantiallycontains no silicon oxide’ is meant that silicon oxide is containedwithin a range that does not affect the property or the characteristicof the present invention, or that silicon oxide is contained in anamount not exceeding the level of the content of impurities. It is notnecessarily meant that the silicon oxide content is below the limit ofdetection.

The zirconia sintered body of the present invention may contain apigment(s) for coloring. If the zirconia sintered body is applied as adental material, chromium oxide (Cr₂O₃), erbium oxide (Er₂O₃), ironoxide (Fe₂O₃), praseodymium oxide (Pr₆O₁₁) and so forth may be used aspigment(s). Such a pigment(s) may be used also in combination. Thecontents of the pigment(s) may partially be differentiated.

For example, if the zirconia sintered body, used as a dental material,contains chromium oxide, the partial content of chromium oxide in alocal portion containing chromium oxide is preferably not larger than0.001 mass % relative to the total mass of the zirconia and thestabilizer. If the zirconia sintered body, used as the dental material,contains erbium oxide, the partial content of erbium oxide in the localportion containing erbium oxide is preferably not larger than 2 mass %relative to the total mass of the zirconia and the stabilizer. If thezirconia sintered body, used as the dental material, contains ironoxide, the partial content of iron oxide in a local portion containingiron oxide is preferably not larger than 0.1 mass % relative to thetotal mass of the zirconia and the stabilizer. If the zirconia sinteredbody, used as the dental material, contains praseodymium oxide, thepartial content of praseodymium oxide in a local portion containingpraseodymium oxide is preferably not larger than 0.1 mass % relative tothe total mass of the zirconia and the stabilizer.

In an X-ray diffraction pattern, as measured using CuKα rays, of thezirconia sintered body, following the sintering and before ahydrothermal treatment test, a sort of degradation acceleration test, aslater explained, the ratio of height of a peak (referred to below as a‘second peak’) existing in the vicinity of a [11-1] peak derived fromthe monoclinic crystal in the vicinity of 28° of 2θ to the height of apeak (referred to below as a ‘first peak’) existing in the vicinity of a[111] peak derived from the tetragonal crystal in the vicinity of 30° of2θ is preferably not larger than 0.1 and more preferably not larger than0.05. By the way, the above ratio, which is the height of the secondpeak/the height of the first peak′, is referred to below as a ‘peakratio of the monoclinic crystal’.

In the zirconia sintered body of the present invention, progress of thephase transition from the tetragonal crystal to the monoclinic crystalis suppressed even though the hydrothermal treatment test is carriedout. For example, in case the zirconia sintered body is hydrothermallytreated at 180° C. and 1 MPa for 5 hours, the peak ratio of themonoclinic crystal in the X-ray diffraction pattern, as measured withCuKα rays on the surface of the hydrothermally treated zirconia sinteredbody, is preferably not larger than unity, more preferably not largerthan 0.8, more preferably not larger than 0.7 and further preferably notlarger than 0.6.

In the present description, the ‘hydrothermal treatment test’ denotes atest pursuant to ISO13356, in which the condition prescribed in ISO13356is ‘134° C., 0.2 MPa, 5 hours’. In the present invention, to make thetest condition more severe, the former two conditions are set at ‘180°C., 1 MPa’, and the test time is appropriately in accordance with agiven objective. The hydrothermal treatment test is also termed a ‘lowtemperature deterioration acceleration test’ or a ‘hydrothermaldeterioration test’.

The flexural strength as measured pursuant to JISR1601 of the zirconiasintered body according to the present invention is preferably not lessthan 1000 MPa, more preferably not less than 1100 MPa and furtherpreferably not less than 1200 MPa. It is noted that these values arethose for the state of the sintered body that is not applied to thehydrothermal treatment test yet.

In the zirconia sintered body according to the present invention, theabove mentioned flexural strength can be obtained in the three-pointbending test even in a case where the load point is located at aposition of the interlayer boundary (may be simply termed as “boundary”herein) in the fabrication method as later explained. FIG. 1schematically depicts a three-point bending test. For example, in thetest sample, the interlayer boundary, which is produced by laminatingzirconia powders of different compositions, is disposed at the center ofthe length (the midpoint in a longitudinal direction) of the testsample. The boundary extends along a direction of load application(along a direction of the smallest cross-sectional area) to traverse thetest sample. The load point in the three-point bending test is alignedwith the position of the boundary. Even in case the flexural strength ismeasured by a test which imposes a load on the boundary, it is possibleto obtain a strength comparable to that of the sintered body which isnot of a laminated (multi-layered) structure, i.e., a sintered body freeof the boundary. For example, in the sintered body according to thepresent invention, the flexural strength measured as load is applied tothe interlayer boundary is preferably not less than 90% and morepreferably not less than 95% of the flexural strength of a local portionother than the boundary, (for example, the flexural strength of apre-sintered body prepared from a non-laminated composition, undercomparable conditions, e.g., same pre-sintering temperaturepre-sintering time).

The fracture toughness of the zirconia sintered body according to thepresent invention, as measured pursuant to JISR1607, is preferably notless than 3.5 MPa·m^(1/2), more preferably not less than 3.8MPa·m^(1/2), more preferably not less than 4 MPa·m^(1/2) and furtherpreferably not less than 4.2 MPa·m^(1/2). By the way, these values arethose obtained in the state prior to performing the hydrothermaltreatment test.

In a test for measuring the fracture toughness of the zirconia sinteredbody, according to the present invention, even in case a load point ison the portion corresponding to the interlayer boundary of the layers inthe fabrication method as later explained, the above mentioned value ofthe fracture toughness may be obtained. For example, in the test sample,the boundary produced by laminating zirconia powders of differentcompositions is located at the center of the test sample (the midpointin the longitudinal direction). The boundary extends along the loadapplying direction (along a direction with the smallest[cross-sectional] area direction) to traverse the test sample. Theposition of a diamond pressing tip used in the measurement test isaligned with the boundary. Even in case where the fracture toughness ismeasured by a test which imposes a load on the boundary, in this manner,it is possible to obtain a fracture toughness comparable to that for anon-laminated, that is, boundary-free, sintered body.

It is desirable for the zirconia sintered body of the present inventionthat the above values are satisfied for every item of the peak ratio ofthe monoclinic crystal after the hydrothermal treatment, flexuralstrength and the fracture toughness. For example, with the zirconiasintered body of the present invention, preferably the peak ratio of themonoclinic crystal after the hydrothermal treatment is not larger thanunity (one), the fracture toughness is not less than 3.5 MPa·m^(1/2),and the flexural strength is not less than 1000 MPa. More preferably,with the zirconia sintered body of the present invention, the peak ratioof the monoclinic crystal after the hydrothermal treatment is not largerthan 0.6, the fracture toughness is not less than 4 MPa·m^(1/2), whilethe flexural strength is not less than 1000 MPa.

In case where the zirconia sintered body according to the presentinvention is colored, in particular the zirconia sintered body graduallychanges in color, i.e., presents color gradation, in one direction, itis desirable that there is a direction along which the color issubstantially not changed. FIG. 2 depicts a schematic illustration forthe zirconia sintered body. In the zirconia sintered body 10, shown inFIG. 2, it is desirable that the color is substantially not changed in afirst direction X. It is assumed that, between optional two points on astraight line extending in the first direction X, the differences inchromaticity values L*, a*, b*, representing the chromaticity values inthe L*a*b* color chromaticity diagram (JISZ8729), are denoted ΔL*, Δa*and Δb* and ΔE*ab is calculated in accordance with the followingequation, ΔE*ab is preferably less than unity (one) and more preferablyless than 0.5.

ΔE*ab=√{square root over ((ΔL*)²+(Δa*)²+(Δb*)²)}{square root over((ΔL*)²+(Δa*)²+(Δb*)²)}{square root over((ΔL*)²+(Δa*)²+(Δb*)²)}  [Equation 2]

In case where the zirconia sintered body of the present invention iscolored, it is desirable that the sintered body changes in color, thatis, presents color gradation, from one end to the opposite end. On astraight line extending in the second direction Y from one end P to theopposite end Q of the zirconia sintered body 10 shown in FIG. 2, theincreasing or decreasing tendency of the L* value, a* value and b* valueis desirably not changed in the reverse direction. Viz., if, on thestraight line extending from the one end P to the opposite end Q, the L*value tends to increase, it is desirable that there exists no domainwhere the L* value substantially decreases. For example, if, on thestraight line extending from the one end P to the opposite end Q, the L*value tends to increase, it is desirable that there exists no domainwhere the L* value decreases by not less than unity, while it is moredesirable that there exists no domain where the L* value decreases bynot less than 0.5. If, on the straight line extending from the one end Pto the opposite end Q, the a* value tends to decrease, it is desirablethat there exists no domain where the a* value substantially increases.For example, if, on the straight line extending from the one end P tothe opposite end Q, the a* value tends to decrease, it is desirable thatthere exists no domain where the a* value increases by unity or more,while it is more desirable that there exists no domain where the a*value increases by 0.5 or more. In addition, if, on the straight lineextending from the one end P to the opposite end Q, the b* value tendsto decrease, it is desirable that there exists no domain where the b*value substantially increases. For example, if, on the straight lineextending from the one end P to the opposite end Q, the b* value tendsto decrease, it is desirable that there exists no domain where the b*value increases by unity or more, while it is more desirable that thereexists no domain where the a* value increases by 0.5 or more.

As for the color change direction in the zirconia sintered body 10, ifthe L* value tends to increase from the one end P to the opposite end Q,it is preferred that the a* and b* values tend to decrease. If, forexample, the zirconia sintered body 10 is used as a dental prostheticmaterial, it is preferred that the color changes from pale yellow, paleorange or thin brown to white from the one end P to the opposite end Q.

Referring to FIG. 2, points on the straight line interconnecting onepoint P and the opposite end Q are labeled a first point A, a secondpoint B, a third point C and a fourth point D, looking from the end P inorder. For example, if the zirconia sintered body 10 is used as dentalprosthesis, the first point A is desirably in a domain of 25% to 45% ofa length from the one point P to the opposite end Q (referred to belowas ‘total length’) as measured from the one end P. The second point B isdesirably in a domain from a site spaced a distance equal to 30% of thetotal length apart from the one point P up to a point of 70% from theone end P. The fourth point D is desirably in a domain of 25% to 45% ofthe total length from the opposite end Q. The third point C is desirablyin a domain from a site spaced a distance equal to 30% of the totallength apart from the opposite point Q up to a point of 70% of the totallength from the opposite end Q.

The chromaticity (L*, a*, b*) of the zirconia sintered body 10 in theL*a*b* color chromaticity diagram (JISZ8729) at the first point A,second point B, third point C and the fourth point D is expressed as(L1, a1, b1), (L2, a2, b2), (L3, a3, b3), (L4, a4, b4), respectively. Itis desirable in this case that the following large/small relationship.By the way, the chromaticity of each point may be found by preparing azirconia sintered body of the sole composition corresponding to eachpoint and measuring the chromaticity of each such zirconia sinteredbody.

L1<L2<L3<L4

a1>a2>a3>a4

b1>b2>b3>b4

In case where the zirconia sintered body is applied to a dentalmaterial, L1 is desirably not less than 58.0 and not larger than 76.0.L2 is desirably not less than 62.5 and not larger than 80.5. L3 isdesirably not less than 69.1 and not larger than 82.3. L4 is desirablynot less than 71.8 and not larger than 84.2.

In case where the zirconia sintered body is applied to a dentalmaterial, a1 is desirably not less than −1.6 and not larger than 7.6. a2is desirably not less than −1.8 and not larger than 5.5. a3 is desirablynot less than −2.1 and not larger than 1.6. a4 is desirably not lessthan −2.1 and not larger than 1.8.

In case where the zirconia sintered body is applied to a dentalmaterial, b1 is desirably not less than 5.5 and not larger than 26.7. b2is desirably not less than 4.8 and not larger than 21.8. b3 is desirablynot less than 3.5 and not larger than 16.2. b4 is desirably not lessthan 1.9 and not larger than 16.0.

In case where the zirconia sintered body is applied to a dentalmaterial, preferably L1 is not less than 60.9 and not larger than 72.5,a1 is not less than 0.2 and not larger than 5.9, b1 is not less than11.5 and not larger than 24.9, L4 is not less than 72.2 and not largerthan 79.2, a4 is not less than −1.2 and not larger than 1.7, b4 is notless than 6.0 and not larger than 15.8. More preferably, L1 is not lessthan 63.8 and not larger than 68.9, a1 is not less than 2.0 and notlarger than 4.1, b1 is not less than 17.5 and not larger than 23.4, L4is not less than 72.5 and not larger than 74.1, a4 is not less than −0.2and not larger than 1.6, b4 is not less than 10.1 and not larger than15.6. This allows matching to the average color tone of teeth.

The color difference ΔE*ab between two neighboring points may beexpressed by the following equation. ΔL* is the difference between theL* values of two neighboring layers, such as (L1-L2). Δa* is thedifference between the a* values of two neighboring layers, such as(a1-a2). Δb* is the difference between the b* values of two neighboringlayers, such as (b1-b2). If the color difference between the first pointA and the second point B is ΔE*ab1, that between the second point B andthe third point C is ΔE*ab2 and that between the third point C and thefourth point D is ΔE*ab3, and the above mentioned relationship holds asto the chromaticity of each of the first point A, second point B, thirdpoint C and the fourth point D, then ΔE*ab1, for example, is desirablynot less than 3.7 and not larger than 14.3. ΔE*ab2 is desirably not lessthan 1.8 and not larger than 17.9. ΔE*ab3 is desirably not less than 1.0and not larger than 9.0. This can reproduce color changes similar tothose of a natural tooth.

ΔE*ab=√{square root over ((ΔL*)²+(Δa*)²+(Δb*)²)}{square root over((ΔL*)²+(Δa*)²+(Δb*)²)}{square root over((ΔL*)²+(Δa*)²+(Δb*)²)}  [Equation 3]

Assumed that the color difference between the first point A and thefourth point D is ΔE*ab4, and the above mentioned relationship holds asto the chromaticity of each of the first point A, second point B, thirdpoint C and the fourth point D, then ΔE*ab4, for example, is desirablynot larger than 36. A value obtained by deducting the color differenceΔE*ab4 between the first point A and the fourth point D from the sum ofthe color difference ΔE*ab1 between the first point A and the secondpoint B, color difference ΔE*ab2 between the second point B and thethird point C and the color difference ΔE*ab3 between the third point Cand the fourth point D is desirably not larger than unity. This allowsrepresenting natural changes in color.

In a case where continuous changes in b* value in the L*a*b* colorchromaticity diagram (JISZ8729) along a straight line traversing thelayers of the powders of respective different compositions (see thefabrication method below) as intersecting the interlayer boundary, thatis, along a second direction Y shown in FIG. 2, are measured, it ispreferred that, even in a direction traversing the layers, the b* valueis not constant and shows a tendency to increase or decrease moderately.It is moreover preferred that, even if in a direction traversing theinterlayer boundary portion, the b* value does not increase or decreaseacutely. The changes in the b* value can be measured using e.g., atwo-dimensional colorimeter manufactured and sold by PaPaLaB Co. Ltd. Inmeasurement, the interval between neighboring measurement points may beset to 13 μm, as an example.

If the zirconia sintered body according to the present invention isapplied as a dental material, the chromaticity of the fourth point D isin the above range, a zirconia sintered body is prepared from solely thecomposition corresponding to the fourth point, and both surfaces of thesintered body are polished to a mirror surface to provide a sample of0.5 mm in thickness, the optical transmittance of the so preparedsample, as measured pursuant to JISK7361, is desirably not less than27%. If the chromaticity of the first point A is in the above range, azirconia sintered body is prepared from solely the compositioncorresponding to the first point, and both surfaces of the sintered bodyare polished to a mirror surface to a sample of 0.5 mm in thickness, theoptical transmittance of the so prepared sample, as measured pursuant toJISK7361, is desirably not less than 10%.

In a case where the zirconia sintered body 10 of the present inventionis applied to the dental material, it is desirable that a length L ofthe zirconia sintered body 10 in a first direction Y satisfies a lengthcorresponding to at least an exposed portion of a natural tooth. Forexample, the length L of the zirconia sintered body 10 is preferably 5mm to 18 mm.

The composition as well as the pre-sintered body for the preparation ofthe zirconia sintered body of the present invention will now beexplained. The composition as well as the pre-sintered body is aprecursor (partly-finished product) of the zirconia sintered body of thepresent invention. The pre-sintered body is obtained on firing, that is,pre-sintering (may be termed “calcining”, too) at a temperature belowthe sintering temperature. The pre-sintered body encompasses a shapedproduct. A dental prosthesis, such as a crown, obtained by milling,grinding and/or cutting a pre-sintered zirconia disc using the CAD/CAM(Computer-Sided Design/Computer-Aided Manufacturing) may be included inthe pre-sintered body.

The composition as well as the pre-sintered body is prepared as zirconiapowders of respective different compositions are laminated one onanother.

Each of the composition and the pre-sintered body contains zirconiacrystal powders, mainly of the monoclinic system, a stabilizer(s) andtitanium oxide. An aluminum oxide may be contained in the composition,too. Preferably, aluminum oxide is αalumina.

The average particle size of zirconia powder (in granulated state) inthe composition is preferably 20 μm to 40 μm.

As the stabilizer(s) contained in the composition as well as thepre-sintered body, oxides, such as calcium (CaO), magnesium oxide (MgO),yttria or cerium oxide (CeO₂) may be given. Preferably, thestabilizer(s) is added in such an amount as to allow the zirconia powderin the sintered body to be partially stabilized. If, for example, yttriais used as the stabilizer, the content of yttria is preferably 2.5 mol %to 4.5 mol %, more preferably 3 mol % to 4.5 mol % and furtherpreferably 3.5 mol % to 4.5 mol %, relative to the total of mols ofzirconia and yttria.

The content of aluminum oxide in the composition as well as thepre-sintered body is preferably 0 mass % (no aluminum oxide content) to0.3 mass % relative to the total mass of the zirconia crystal particlesand the stabilizer(s) in order to elevate the strength of the zirconiasintered body. If the content of aluminum oxide exceeds 0.3 mass %,transmittance of the zirconia sintered body is lowered.

The content of titanium oxide in the composition as well as thepre-sintered body is preferably 0 mass % (no titanium oxide content) to0.6 mass % relative to the total mass of the zirconia crystal particlesand the stabilizer(s) in order to promote growth of zirconia crystalgrains. If the content of titanium oxide exceeds 0.6 mass %, strength ofthe zirconia sintered body is lowered.

The content of silicon oxide in the composition as well as thepre-sintered body is preferably 0.1 mass % or less relative to the totalmass of the zirconia crystal particles and the stabilizer(s).Preferably, the composition as well as the pre-sintered body issubstantially free of silicon oxide SiO₂ (silica). It is because thecontent of silicon oxide lowers the transmittance of the zirconiasintered body. By the phrase ‘substantially free of silicon oxide’ it ismeant that silicon oxide is contained within a range not affecting theproperty or the characteristic of the present invention, or that siliconoxide is preferably contained in an amount not exceeding the level ofthe content of impurities. It is not necessarily meant that the siliconoxide content is to be lower than the limit of detection.

The composition as well as the pre-sintered body according to thepresent invention may contain a pigment(s) for coloring. If the zirconiasintered body, prepared from the composition or the pre-sintered body,is used as the dental material, chromium oxide (Cr₂O₃), erbium oxide(Er₂O₃), iron oxide (Fe₂O₃), praseodymium oxide (Pr₆O₁₁) and so forthmay be used as pigments, either alone or in combination. The contents ofthe pigments may partially be differentiated.

If the shaped composition or the pre-sintered body in its entirety isdivided into four layers, a local portion from the bottom end to 25% to45% of the total thickness is a first layer, a local portion from thetop of the first layer to 5% to 25% of the total thickness is a secondlayer, a local portion from the top of the second layer to 5% to 25% ofthe total thickness is a third layer and a local portion from the top ofthe third layer to an upper end, having a thickness corresponding to 25%to 45% of the total thickness, is a fourth layer, preferably the pigmentcontent decreases from the first layer towards the fourth layer.

If a sintered body, prepared from the composition or the pre-sinteredbody, is used as a dental material, erbium oxide and iron oxide may beadded as pigment(s). In this case, the content of erbium oxide and thecontent of iron oxide in the first layer relative to the total mass ofthe zirconia and the stabilizer are preferably 0.33 mass % to 0.52 mass% and 0.05 mass % to 0.12 mass %, respectively. The content of erbiumoxide and the content of iron oxide in the second layer relative to thetotal mass of the zirconia and the stabilizer are preferably 0.26 mass %to 0.45 mass % and 0.04 mass % to 0.11 mass %, respectively. The contentof erbium oxide and the content of iron oxide in the third layerrelative to the total mass of the zirconia and the stabilizer arepreferably 0.05 mass % to 0.24 mass % and 0.012 mass % to 0.08 mass %,respectively. The content of erbium oxide and the content of iron oxidein the fourth layer relative to the total mass of the zirconia and thestabilizer are preferably 0 mass % to 0.17 mass % and 0 mass % to 0.07mass %, respectively. Preferably, the content of erbium oxide and thecontent of iron oxide decrease from the first layer towards the fourthlayer in order.

If, for example, a sintered body prepared from a composition or apre-sintered body is used as a dental material, erbium oxide, iron oxideand chromium oxide may be added as the pigments. If the sintered bodyprepared from the composition or the pre-sintered body is used as adental material, it is preferred that, in the first layer, the contentof erbium oxide, that of iron oxide and that of chromium oxide relativeto the total mass of the zirconia and the stabilizer(s) are preferably0.08 mass % to 0.37 mass %, 0.08 mass % to 0.15 mass % and 0.0008 mass %to 0.0012 mass %, respectively. In the second layer, it is preferredthat the content of erbium oxide, that of iron oxide and that ofchromium oxide relative to the total mass of the zirconia and thestabilizer(s) are preferably 0.06 mass % to 0.42 mass %, 0.06 mass % to0.18 mass % and 0.0006 mass % to 0.001 mass %, respectively. In thethird layer, it is preferred that the content of erbium oxide, that ofiron oxide and that of chromium oxide relative to the total mass of thezirconia and the stabilizer(s) are preferably 0.06 mass % to 0.17 mass%, 0.018 mass % to 0.042 mass % and 0.0001 mass % to 0.0003 mass %,respectively. In the fourth layer, it is preferred that the content oferbium oxide, that of iron oxide and that of chromium oxide relative tothe total mass of the zirconia and the stabilizer(s) are preferably 0mass % to 0.12 mass %, 0 mass % to 0.001 mass % and 0 mass % to 0.0001mass %, respectively. It is preferred that the content of erbium oxide,that of iron oxide and that of chromium oxide decrease from the firstlayer towards the fourth layer in order.

If, for example, a sintered body prepared from the composition or thepre-sintered body is used as a dental material, erbium oxide, iron oxideand praseodymium oxide may be added as the pigments. If the sinteredbody prepared from the composition or the pre-sintered body is used as adental material, it is preferred that, in the first layer, the contentof erbium oxide, that of iron oxide and that of praseodymium oxiderelative to the total mass of the zirconia and the stabilizer(s) arepreferably 0.08 mass % to 2.2 mass %, 0.003 mass % to 0.12 mass % and0.003 mass % to 0.12 mass %, respectively. In the second layer, it ispreferred that the content of erbium oxide, that of iron oxide and thatof praseodymium oxide relative to the total mass of the zirconia and thestabilizer(s) are preferably 0.06 mass % to 1.9 mass %, 0.002 mass % to0.11 mass % and 0.002 mass % to 0.11 mass %, respectively. In the thirdlayer, it is preferred that the content of erbium oxide, that of ironoxide and that of praseodymium oxide relative to the total mass of thezirconia and the stabilizer(s) are preferably 0.018 mass % to 1 mass %,0.008 mass % to 0.06 mass % and 0.0008 mass % to 0.06 mass %,respectively. In the fourth layer, it is preferred that the content oferbium oxide, that of iron oxide and that of praseodymium oxide relativeto the total mass of the zirconia and the stabilizer(s) are preferably 0mass % to 0.7 mass %, 0 mass % to 0.05 mass % and 0 mass % to 0.05 mass%, respectively. It is preferred that the content of erbium oxide, thatof iron oxide and that of praseodymium oxide decrease from the firstlayer towards the fourth layer in order.

The content of the pigment(s) can be theoretically calculated from theamount of its addition with respect to the total mass of the zirconiaand the stabilizer(s) and from the fabrication method.

The flexural strength of the pre-sintered body of the present invention,as measured in accordance with JISR1601, is preferably not less than 38MPa, more preferably not less than 40 MPa and further preferably notless than 42 MPa.

With the pre-sintered body according to the present invention, even if,in the three-point bending test, the load point is located at aninterlayer boundary portion produced by laminating zirconia powders ofdiffering compositions (see the fabrication method below), the abovementioned flexural strength can be obtained. If the flexural strength ismeasured by the same bending test as that for the above mentionedsintered body, a flexural strength higher than that of the pre-sinteredbody prepared by simply laminating of powders (without applyingvibration, for example) may be obtained. If the flexural strength ismeasured by a test which imposes load on the interlayer boundary, suchstrength comparable with that of a non-laminated, that is, interlayerboundary-free, pre-sintered body, may be obtained. With the pre-sinteredbody according to the present invention, the flexural strength measuredunder a load applied to the interlayer boundary is preferably not lessthan 90% and more preferably not less than 95% of the flexural strengthas measured at a local portion other than the interlayer boundary, forexample, the flexural strength of a pre-sintered body prepared from anon-laminated composition under comparable conditions, such as the samepre-sintering temperature and pre-sintering time.

With the composition and the pre-sintered body according to the presentinvention, even if heating is performed for pre-sintering or sintering,no layer exfoliation occurs at the boundary between lamination layers ofzirconia powders of respective different compositions. In addition, theoverall deformation can be suppressed. FIG. 3 and FIG. 4 depictschematic views of a test sample used for measuring the extent of thedeformation at the time of sintering. FIG. 3 shows a schematic drawingof a two-layered lamination body. FIG. 4 shows a schematic drawing of afour-layered lamination body. FIG. 5 depicts a schematic view forillustrating a method for measuring the extent of the deformation. As anexample, a plurality of zirconia powders, having respective differentcompositions, are laminated together to form a composition, and thecomposition is then fired (calcined) at 800° C. to 1200° C. for twohours to form a pre-sintered body. The pre-sintered body is then shapedby the CAD/CAM system to a rectangular parallelepiped which is 50 mm inwidth[length], 10 mm in height and 5 mm in depth[thickness], as shown inFIG. 3 and FIG. 4. This serves as a test sample. As an example, a testsample 20 that is a two-layered lamination body, shown in FIG. 3, has afirst layer 21 a and a second layer 21 b. Each of the thickness of thefirst layer 21 a and that of the second layer 21 b accounts for 50% ofthe total thickness. A test sample 22 that is a four-layered laminationbody, shown in FIG. 4, includes a first layer 23 a, a second layer 23 b,a third layer 23 c and a fourth layer 23 d. The thickness of the firstlayer 23 a and that of the fourth layer 23 d each account for 35% of thetotal thickness. The thickness of the second layer 23 b and that of thethird layer 23 c each account for 15% of the total thickness. If, ineach of the test samples 20, 22, the surface of 50 mm by 5 mm issupposed to be the bottom surface (upper or lower surface), each layerextends in the same direction as, preferably parallel to, the bottomsurfaces 20 a, 22 a. That is, each interlayer boundary is parallel tothe bottom surfaces 20 a, 22 a. If the test samples are fired at 1500°C. for two hours for sintering, the test samples are deformed so thatthe bottom surfaces 20 a, 22 a are flexed. The test samples 20, 22 areset on a flat surface (ground surface 30) with the concave surface sidedirected downwards. The width of each of the as-deformed test samples20, 22, that is, a distance L between the ground contacting points(fulcrum points) along the width, is measured. On the other hand, a gapd at the largest portion between the bottom surfaces 20 a, 22 a deformedto the concave shape and the ground surface 30 is measured. The extent(amount) of the deformation is calculated as (d/LX100). The deformationis preferably not larger than 0.15, more preferably not larger than 0.1,more preferably not larger than 0.05 and further preferably not largerthan 0.03.

Such a composition or a pre-sintered body, obtained on laminatingzirconia powders of different compositions, is susceptible todeformation when subjected to sintering. With the composition or thepre-sintered body according to the present invention, the extent of thedeformation can be made smaller than that in the composition or thepre-sintered body obtained on simple lamination. As a result, an endproduct can be improved in dimensional accuracy. The composition and thepre-sintered body according to the present invention can be applied toadvantage to a dental prosthesis that may appreciably be different fromperson to person. By the way, a mixture layer presumed to have beenformed on a boundary (interface) between neighboring (upper and lower)layers is not shown in FIG. 3 or in FIG. 4 for simplicity.

The composition of the present invention may be powder, a fluid obtainedon adding powders to a solvent, or a shaped body obtained on shaping thepowders to a preset shape. That is, the composition may be powdery, orpaste-like or wet composition. (In other words, the composition may bepresent in a solvent or contain a solvent.) The composition may alsocontain an additive(s), such as a binder(s) and pigment(s). By the way,the mass of the solvent and the additive such as the binder is not takeninto account in calculating the content ratio.

In case where the composition of the present invention is a shaped body,there is no limitation to the method of shaping. The composition may beshaped by e.g., pressing, injection molding or stereolithography (oropto-molding). It may also be shaped by multistage shaping (forming).For example, the composition of the present invention may be shaped bypressing followed by cold isostatic pressing (CIP).

The pre-sintered body according to the present invention can be obtainedby firing the composition of the present invention at 800° C. to 1200°C. under normal atmospheric pressure.

The pre-sintered body of the present invention can be adapted to formthe zirconia sintered body according to the present invention by beingfired at 1350° C. to 1600° C. under normal atmospheric pressure.

The length of the composition and the pre-sintered body along itslaminating (i.e., layer-stacking) direction (thickness) is preferablydetermined so as to realize a targeted length of the sintered body assintering shrinkage is taken into account. When the sintered bodyprepared from the composition or the pre-sintered body is used as adental material, as an example, the targeted length along the laminatingdirection of the sintered body is 5 mm to 18 mm, as an example, whilethe length (thickness) along the laminating direction of the compositionor the pre-sintered body may be set at 10 mm to 26 mm.

An example of a fabrication method for the composition and thepre-sintered body as well as the sintered body according to the presentinvention will now be explained. Here, the method for gradually changingthe color of the sintered body (color gradation) will also be explained.

Initially, zirconia and the stabilizer are wet-mixed together in waterto form a slurry. Next, the slurry is then dried and granulated. Theresulting granules are then pre-sintered to form primary powders.

If color gradation is to be imparted to the sintered body, the primarypowders are divided into two groups. Then, a pigment(s) is added to atleast one of the two groups of the primary powders to provide fordifference in the ratio of pigment addition. For example, a pigment maybe added to one of the groups, whereas no pigment may be added to theother. A powder (powder group) with a low addition ratio of a specifiedpigment is referred to below as “low addition ratio powder”, while apowder (powder group) with a high addition ratio of the specifiedpigment is referred to below as “high addition ratio powder”. The amountof pigment addition in the high addition ratio powder is preferablyadapted to an addition ratio of a portion having darkest color in thesintered body. With regard to each powder, zirconia is mixed andpulverized in water to a desired particle size to form a zirconiaslurry. Next, the slurries are dried and granulated to form secondarypowders. In case of addition of additive(s) such as aluminum oxide,titanium oxide and the binder, they may be added at the time ofpreparation of the primary powders or at the time of preparation of thesecondary powders.

A plurality of powders having respective different pigment contents arethen prepared from the secondary powders of the low addition ratiopowder and the high addition ratio powder. As an example, if thefour-layered lamination composition and pre-sintered body are to beprepared, a first powder for the first layer may be made up of 100% ofthe high addition ratio powder without adding the low addition ratiopowder. A second powder for the second layer may be prepared by mixingthe low addition ratio powder and the high addition ratio powder at amixing ratio of the low addition ratio powder to high addition ratiopowder of 5:95 to 15:85. A third powder for the third layer may beprepared by mixing the low addition ratio powder and the high additionratio powder at a mixing ratio of the low addition ratio powder to highaddition ratio powder of 35:65 to 45:55. A fourth powder for the fourthlayer may be prepared by mixing the low addition ratio powder and thehigh addition ratio powder at a mixing ratio of the low addition ratiopowder to high addition ratio powder of 45:55 to 55:45. By way ofalternative values of the mixing ratio, in preparing the above mentionedfour-layered lamination composition and pre-sintered body, the firstpowder for the first layer may be made up of 100% of the high additionratio powder without adding the low addition ratio powder. The secondpowder for the second layer may be prepared by mixing the low additionratio powder and the high addition ratio powder at a mixing ratio of thelow addition ratio powder to high addition ratio powder of 10:90 to30:70. The third powder for the third layer may be prepared by mixingthe low addition ratio powder and the high addition ratio powder at amixing ratio of the low addition ratio powder to high addition ratiopowder of 70:30 to 90:10. The fourth powder for the fourth layer may bemade up of 100% of the low addition ratio powder without adding the highaddition ratio powder.

In using the zirconia sintered body as the dental material, thedifference between the mixing ratios for the second and third layers ispreferably larger than that between the mixing ratios for the first andsecond layers as well as that between the mixing ratios for the thirdand fourth layers. By so doing, it is possible to reproduce colorchanges comparable to those of a natural tooth.

By adjusting the pigment contents in the respective layers, based on thetwo sorts of powders that present different colors in the sintered body,it is possible to realize natural changes in color (color gradation) bylaminating the respective powders (stacking layers) in order.

If laminating is made for some other objective than coloring, thesecondary powders may be divided into a number of groups correspondingto the number of the layers. A desired additive(s) may be added to eachpowder.

A plurality of powders having respective different pigment contents arethen laminated in order. If desired to impart color gradation to thesintered body, it is preferred that the powders are laminated so thatthe ratio of addition of a particular pigment becomes higher or lowerstepwise in the sequence of layering. Initially, powder(s) of the firstlayer is charged into a mold and an upper surface of the powder(s) ofthe first layer is made flat. As a way of making it flat the uppersurface of the powder(s), vibrating the mold or leveling the uppersurface of the powder(s) of the first layer may be adopted. It ispreferred not to perform pressing until the totality of the layers hasbeen laminated. The powder(s) of the second layer is then charged on topof the powder(s) of the first layer. The mold is then vibrated so thatthe vibration is transmitted to the powders in the mold. As a way ofgiving the vibration, a desired way, such as giving a mechanicalvibration to the mold, vibrating (or swinging) the mold manually andstriking the mold with a hammer, for example, may be suitably adopted.It is thought that, by so doing, the powder(s) of the first layer andthat of the second layer are partially mixed together at a boundarybetween the powders of the first and second layers. The number of timesas well as intensity of the vibrations and, in the case of mechanicalvibrations, the frequency and amplitude of the vibration may beappropriately set, depending on the particle size, particle sizedistribution or the particle shape, so that mixing of the powders of theupper and lower layers will take place on the interlayer boundary. Theupper surface of the powder(s) of the second layer is then made flat asin the case of the powder(s) of the first layer. The sequence ofoperations is repeated until all of the layers are laminated.

If the above mentioned four-layered composition and pre-sintered bodyare to be formed, the first powder(s) is charged to a predeterminedthickness, such as to 25% to 45% of the overall thickness. At this time,the upper surface of the first powder(s) is made flat, but pressing isnot performed. The second powder(s) is then charged on the firstpowder(s) to a predetermined thickness, such as to 5 to 25% of theoverall thickness. The mold is then vibrated. It is presumed that thisvibration forms a first boundary layer, which is a mixture of the firstand second powders, at a boundary between an upper surface of the firstpowder(s) and a lower surface of the second powder(s). The upper surfaceof the second powder is then made flat. Pressing is not applied to thesecond powder before charging the third powder. The third powders arecharged on the second powder to a predetermined thickness, for example,to 25% to 45% of the overall thickness. The mold is then vibrated. It ispresumed that this vibration forms a second boundary layer, which is amixture of the second and third powders, at a boundary between an uppersurface of the second powder(s) and a lower surface of the thirdpowder(s). The upper surface of the third powder(s) is then made flat.Pressing the third powder(s) is not performed before charging the fourthpowder(s). The fourth powder(s) is charged on the third powder(s) to apredetermined thickness, for example, to 25% to 45% of the overallthickness. The mold is then vibrated. It is presumed that this vibrationforms a third boundary layer, which is a mixture of the third and fourthpowders, at a boundary between an upper surface of the third powder(s)and a lower surface of the fourth powder(s).

After laminating the entire layers, pressing is carried out to form ashaped product as the composition of the present invention. The shapedproduct may then be subjected to CIP.

It is thought that, by not applying pressing before charging powder(s)of the next layer, and by applying vibration each time each layer ischarged, a boundary layer where powders of upper and lower layers aremixed can be formed between neighboring layers. This enhances adhesiontightness between neighboring layers in the sintered body. The extent orspeed of shrinkage at the time of heating may be equalized with that ofeach layer to prevent layer exfoliation at the time of heating orirregular deformation of the sintered body from the targeted shape. Inaddition, since the color difference between neighboring layers may bemoderated, color change can occur naturally along the laminatingdirection in the sintered body (color gradation can be created).

Moreover, in this method, there is no necessity to provide anintermediate layer between main layers. That is, when four main layersare to be laminated, it is only necessary to laminate only the fourlayers. Additionally, pressing is not needed for each layer.Accordingly, work and time can be significantly reduced, and thusmanufacturing cost can be reduced.

In case where no pre-sintered body is fabricated, the composition isfired at 1400° C. to 1600° C. and preferably at 1450° C. to 1550° C. tosinter the zirconia powder(s) to fabricate the zirconia sintered bodyaccording to the present invention. Shaping to a desired shape may beperformed in a stage of the shaped product.

In case where a pre-sintered body is fabricated, the composition isfired at 800° C. to 1200° C. to form a pre-sintered body. Thepre-sintered body is then fired at 1400° C. to 1600° C., preferably1450° C. to 1550° C. to sinter the zirconia powder to fabricate thezirconia sintered body according to the present invention. Shaping maybe performed by milling, grinding and/or cutting etc. in a stage of thepre-sintered body or following the sintering. The shaping may be carriedout with the CAD/CAM system.

The fabrication method for a dental prosthesis is similar to the abovedescribed fabrication method except that the pre-sintered or sinteredbody is shaped to the form of a crown.

In the above described exemplary embodiment, the composition,pre-sintered body and the sintered body, in the form of a four-layeredlamination structure, has been shown and explained. However, the numberof layers may be other than four. The composition, pre-sintered body orthe sintered body may, as an example, be formed with two layers, namelythe first and fourth layers. Alternatively, the composition,pre-sintered body or the sintered body may, as an example, be formedwith three layers, namely the first, second and fourth layers. It isnoted that FIG. 2 is only for facilitated explanation of the positionalrelationships and directions of respective points such that the shapeand size are not limited to those shown in FIG. 2.

EXAMPLES Examples 1 to 4 [Preparation of Composition, Pre-Sintered BodySample and Sintered Body Sample]

A sintered body sample was fabricated from a composition prepared onlaminating zirconia powders of respective different compositions andmeasurement was made of its flexural strength, chromaticity and extentof deformation.

Initially, a zirconia powder containing a stabilizer was prepared. 7.2mass % (4 mol %) of yttria, as stabilizer, were added to 92.8 mass % ofmainly monoclinic zirconia powder. An alumina sol was added so that theamount of addition of alumina is 0.1 mass % to the powder mixture ofzirconia and yttria (100 mass %). Then, 150 mass % of water, 0.2 mass %of an anti-foaming agent and 1 mass % of a dispersant were added to thepowder mixture of zirconia and yttria (100 mass %). The resultingmixture was pulverized with a ball mill for 10 hours. The averageparticle size of a slurry obtained by pulverization (ballmilling) was0.12 μm. The slurry was granulated, using a spray drier, and so formedgranules were pre-sintered at 1000° C. for two hours to prepare primarypowder.

Next, the primary powder was divided into two groups, and a pigment wasadded to at least one of the groups. The powder of the group with a lowpigment addition ratio is termed a “low addition ratio powder”, and thatof the group with a high pigment addition ratio is termed a “highaddition ratio powder”. Table 1 shows the addition ratios of Examples 1to 3. Table 4 shows the addition ratio of Example 4. The values shown inTables 1 and 2 are those of addition ratios related to the amount of thepowder mixture of zirconia and yttria (100 mass %). In each powder, 0.2mass % of titania, 200 mass % of water, 0.2 mass % of an anti-foamingagent and 1 mass % of a dispersant were added to the powder mixture ofzirconia and yttria (100 mass %). Each resulting mixture was pulverizedwith a ballmill for 15 hours. The average particle size of the slurryafter the pulverization was 0.13 μm. Then, 6 mass % of the binder and0.5 mass % of a mold release agent were added to the slurry and mixedwith a ball mill for 15 minutes. The resulting slurry was granulated bya spray drier to form secondary powders of the low addition ratio powderand the high addition ratio powder.

The low addition ratio powder and the high addition ratio powder werethen mixed in ratios shown in Tables 3 to 6 to form first to fourthpowders.

A shaped body sample was then prepared. In Examples 1, 3 and 4, 35 grsof the first powder was charged in a metal mold with an inside size of82 mm by 25 mm and an upper surface of the first powder was swept. 15grs of the second powder was then charged on the first powder and themetal mold was vibrated by a vibrator. An upper surface of the secondpowder was swept off to a flat surface. 15 grs of the third powder wasthen charged on the second powder and the metal mold was vibrated by thevibrator. An upper surface of the third powder was swept off to a flatsurface. 35 grs of the fourth powder was then charged on the thirdpowder and the metal mold was vibrated by the vibrator. An upper surfaceof the fourth powder was swept off to a flat surface. Example 2 wascarried out in the same way as in Examples 1, 3 and 4 except that 50 grsof the first powder and 50 grs of the second powder were charged. Anupper mold was then set and the powder mixture was subjected to primarypress forming at a surface pressure of 200 kg/cm² for 90 seconds using auniaxial pressing apparatus. The primary press shaped body sample wassubjected to CIP shaping at 1500 kg/cm² for five minutes to prepare ashaped body sample.

The shaped body samples were then fired at 1000° C. for two hours toform a pre-sintered body sample. The pre-sintered body samples were thenformed to a shape of a dental crown using the CAD/CAM system (Katanasystem, Kuraray Noritake Dental Inc.). The pre-sintered body sample wasthen fired at 1500° C. for two hours to form a sintered body sample. Thelength of the sintered body along the direction of laminating the firstto fourth powders was 8 mm.

In the sintered body samples of each of the Examples 1 to 4, anappearance resembling a natural tooth was presented, with colorgradation of from pale yellow to yellow-white color from a regioncorresponding to the first layer towards a region corresponding to thefourth layer of the composition.

TABLE 1 Erbium Iron Praseodymium Examples oxide/ oxide/ oxide/ 1 to 3mass % mass % mass % Low addition 0.1 0.005 0.005 ratio powder Highaddition 2 0.1 0.1 ratio powder

TABLE 2 Erbium Iron Praseodymium oxide/ oxide/ oxide/ Example 4 mass %mass % mass % Low addition 0 0 0 ratio powder High addition 2 0.1 0.1ratio powder

TABLE 3 First Second Third Fourth Example 1 powder powder powder powderLow addition  0% 20% 80% 100% ratio powder High addition 100% 80% 20% 0% ratio powder

TABLE 4 First Second Example 2 powder powder Low addition ratio powder 0% 100% High addition ratio powder 100%  0%

TABLE 5 First Second Third Fourth Example 3 powder powder powder powderLow addition  0% 25% 75% 100% ratio powder High addition 100% 75% 25% 0% ratio powder

TABLE 6 First Second Third Fourth Example 4 powder powder powder powderLow addition  0% 25% 75% 100% ratio powder High addition 100% 75% 25% 0% ratio powder

[Measurement of Flexural Strength]

The flexural strength of the pre-sintered body samples and the sinteredbody samples, prepared in Example 4, was measured pursuant to JISR1601.As Comparative Examples, the flexural strength was also measured of thepre-sintered body sample and the sintered body sample, in which novibration was applied to the powders being charged. Comparative Example1 is for a pre-sintered body sample and a sintered body samplefabricated from a composition in which each layer was not pressed at thetime of charging. Comparative Example 2 is for a pre-sintered bodysample and a sintered body sample fabricated from a composition in whicheach layer was pressed at the time of charging. The flexural strengthwas measured pursuant to JISR1601. The test sample was cut out so thatthe longitudinal direction was along the laminating direction. Theboundary between the second and third layers was positioned at thecenter of the test sample, as shown in FIG. 1. The boundary extendedalong the direction of load application, i.e., along a direction of theleast [cross-sectional] area, so as to traverse the test sample. Theflexural strength was measured with the load point of the three-pointbending test aligned with the boundary position. Table 7 shows measuredresults.

The flexural strength of the pre-sintered body sample of Example 4 was40 MPa or more, however, those of the Comparative Examples were notlarger than 36 MPa. From this it is seen that imparting vibration at thetime of laminating of the powders can lead to improved joining strengthbetween the layers at the stage of the pre-sintered body sample. Theflexural strength of the sintered body sample of Example 4 was not lessthan 1200 MPa, however, those of the Comparative Examples 1, 2 were lessthan 1100 MPa, thus lower by 100 MPa or more than in Example 4. It hasturned out that the joining strength between the layers can be elevatedfor the sintered body sample as well.

The flexural strengths of the pre-sintered body sample as well as thesintered body sample of Example 4 were similar to those of the sinteredbody sample fabricated without laminating, as taught in Example 9explained later, and it was found that the laminating did not cause thelowering of the joining strength. It is thus seen that, by impartingvibration at the time of laminating the powders, the interlayer boundaryof the laminated sintered body sample as well as the pre-sintered bodysample exhibits strength equivalent to that of a local area other thanthe boundary.

Primarily, it is presumed that the vibration applied to the powders atthe time of the laminating produces partial mixing of the powders ofupper and lower layers at the interlayer boundary to lead to anincreased joining strength between the layers. Secondarily, it ispresumed that, since the first to fourth powders are fabricated bymixing of the two sorts of powders, the difference in properties of thepowders is only small to lead to an improved affinity in joining.

TABLE 7 Flexural strength Flexural strength of pre-sintered of sinteredSample for measurement body/MPa body/MPa Example 4 (with 41 1219vibration; without pressing) Comparative Example 1 35 1078 (novibration, without pressing) Comparative Example 2 30 1009 (novibration; with pressing

[Measurement of Fracture Toughness]

Fracture toughness was measured of the sintered body sample, fabricatedin Example 4, in accordance with JISR1607. The position of the boundaryin the test sample was the same as in the above mentioned flexuralstrength testing. The position of the pressing tip was aligned with theboundary between the second and third powders. As a result, the fracturetoughness was 4.3 MPa·m^(1/2). This value is similar to that of thefracture toughness of the sintered body sample fabricated withoutlaminating, as taught in Example 9, shown below, thus indicating thatdeterioration in fracture toughness was not produced by laminating.

[Measurement of Shrinkage Deformation at the Time of Sintering]

A test sample(s), described above and shown in FIG. 3 and FIG. 4, wasfabricated from the pre-sintered body sample, fabricated by beingpre-sintered at 1000° C. for two hours as in Example 4, and was fired at1500° C. for two hours to measure the extent of deformation (d/LX100).The extent of the deformation was measured using the above mentionedmeasurement method. As a Comparative Example, the same test sample(s)was prepared for each of Comparative Examples 1 and 2, as in the bendingtest, and the extent of deformation after sintering was measured. Table4 shows test results.

In the Comparative Examples, the extent of the deformation was 0.15 orlarger. In Example 4, the extent of the deformation can be 0.05 or less,indicating that the extent of the deformation can be suppressedappreciably as compared to the Comparative Examples. It is thought fromthis that, by vibrating the composition at the time of layering thepowders of different compositions, it is possible to suppress shrinkagedeformation at the time of the sintering more satisfactorily.

Comparison between Comparative Examples 1 and 2 indicates that theextent of the deformation is smaller in Comparative Example 1. Fromthis, it is thought that not performing pressing after charging eachlayer can lead to more effective suppression of shrinkage deformation atthe time of the sintering.

Comparison between the two-layered body sample and the four-layered bodysample also indicates that the latter is deformed to a lesser extentthan the former. From this, it is thought that an increased number oflayers can lead to more effective suppression of shrinkage deformation.

TABLE 8 Extent of Number of Deformation Samples layers (=d/L × 100)Example 4 (with vibration; 2 0.030 without pressing) 4 0.025 ComparativeExample 1 2 0.285 (without vibration; 4 0.190 without pressing)Comparative Example 2 2 0.500 (withoug vibration; with 4 0.395 pressing)

[Measurement of Chromaticity and Color Difference]

With regard to the first, second, third and fourth powders of Examples 1to 4, sintered body samples of the respective powders alone wereprepared, and chromaticity values of the L*a*b* color chromaticitydiagram were measured. For measurement of the chromaticity values, thesintered body sample was worked to a disc of 14 mm in diameter and 1.2mm in thickness, and both faces of the disc were polished smooth. Adevice for measurement of the chromaticity values, manufactured byOlympus Corporation under the trade name of CE100-DC/US, was used formeasurement. Based on the results of chromaticity measurement, the colordifferences ΔE*ab1 to ΔE*ab3 between respective neighboring layers werecalculated. The color difference ΔE*ab4 between the first and fourthlayers was calculated. In addition, (ΔE*ab1+ΔE*ab2+ΔE*ab3−ΔE*ab4) wascalculated. Tables 9-12 show the chromaticity. Table 13 shows the colordifference.

It is thought that the chromaticity of the sintered body sample of eachpowder represents chromaticity of the color locally presented by thezirconia sintered body sample.

In the sintered body sample of the first layer of the four-layeredlamination body sample, L* was 58 to 73, a* was 0 to 8 and b* was 14 to27. In the sintered body sample of the second layer, L* was 64 to 73, a*was 0 to 6 and b* was 16 to 22. In the sintered body sample of the thirdlayer, L* was 70 to 78, a* was −2 to 2 and b* was 5 to 17. In thesintered body sample of the fourth layer, L* was 72 to 84, a* was −2 to1 and b* was 4 to 15

The color difference between the sintered body sample of the first layerand that of the second layer was 7 to 14. The color difference betweenthe sintered body sample of the second layer and that of the third layerwas 10 to 18. The color difference between the sintered body sample ofthe third layer and that of the fourth layer was 4 to 9. The colordifference between the sintered body sample of the first layer and thatof the fourth layer was 28 to 36. A value obtained by deducting thecolor difference between the sintered body sample of the first layer andthat of the fourth layer from the sum of the color difference betweenthe sintered body sample of the first layer and that of the secondlayer, the color difference between the sintered body sample of thesecond layer and that of the third layer and color difference betweenthe sintered body sample of the third layer and that of the fourth layerwas not larger than unity (one)

TABLE 9 Example 1 L* a* b* Sintered body of fourth 75.70 −1.45 5.68powder Sintered body of third 71.75 −1.15 8.35 powder Sintered body ofsecond 67.75 4.51 16.30 powder Sintered body of first 58.20 7.39 26.13powder

TABLE 10 Example 2 L* a* b* Sintered body of second 75.70 −1.45 5.68powder Sintered body of first 58.20 7.39 26.13 powder

TABLE 11 Example 3 L* a* b* Sintered body of fourth 76.10 −1.42 5.85powder Sintered body of third 71.27 0.86 11.22 powder Sintered body ofsecond 63.03 5.25 21.75 powder Sintered body of first 58.20 7.35 26.50powder

TABLE 12 Example 4 L* a* b* Sintered body of fourth 83.95 −1.87 4.15powder Sintered body of third 77.00 0.25 9.29 powder Sintered body ofsecond 64.12 4.95 20.24 powder Sintered body of first 58.20 7.35 26.50powder

TABLE 13 Examples 1 2 3 4 Color difference between third 4.8 — 7.6 8.9and fourth powders ΔE*ab1 Color difference between 10.5 — 14.1 17.5second and third powders ΔE*ab2 Color difference between first 14.0 28.37.1 8.9 and second powders ΔE*ab3 Color difference between first 28.3 —28.7 35.3 and fourth powders ΔE*ab4 (ΔE*ab1 + ΔE*ab2 + ΔE*ab3) − 1.0 —0.04 0.07 ΔE*ab4

Example 5 Measurement of Change in b* Value

Low addition ratio powder and high addition ratio powder were preparedby adding pigments at the rates shown in Table 14 to a powder mixture ofzirconia and yttria (100 mass %) and a composition was prepared at theproportions shown in Table 15. From the composition, a sintered bodysample was prepared in the same way as in Examples 1 to 4. Change in thevalue of b* of the L*a*b* color chromaticity diagram was measured alongthe laminating direction, that is, along the second direction Y in FIG.2. FIG. 6 depicts a schematic view of the prepared test sample andresults of measurement. Specifically, an upper part of FIG. 6 depicts aschematic view of the test sample, also showing the size and themeasurement direction, and a lower part of FIG. 6 a graph showing theresults of measurement. The as-sintered test sample was fabricated sothat the sample has a size of 20 mm by 20 mm by 1 mm after thesintering. The first layer was a local area where the first powders werecharged, and the fourth layer was a local area where the fourth powderswere charged. The b* value was measured, using a two-dimensionalcolorimeter RC-300 manufactured and sold by PaPaLaB Co. Ltd., as thetest sample was placed at the center of a 29 mm by 22 mm size image,under scanning in a direction perpendicular to the boundaries of therespective layers at an interval of ca. 13 μm. The numerical valuesentered on the x-axis in the graph of the lower part of FIG. 6 indicatethe numbers of measurement points. As a Comparative Example 3, thechange in the value of b* was also measured for a sintered body sampleobtained without the vibration when the powder of each layer waslaminated and with the pressing each time each layer was charged. Thecomposition as well as ratios of the low addition ratio powder and thehigh addition ratio powder in the Comparative Example 3 was the same asthose of Example 5. FIG. 7 depicts a schematic view of the test sampleand the results of measurement.

Referring to the graph of FIG. 7, the value of b* shows a flat profileat a center portion of each layer, and, in the interlayer boundary, thevalue of b* shows step-like acute changes. This is presumably due to thefact that the powders of the respective layers having different pigmentcompositions have been sintered independently of one another. From thisit is seen that there lacks tidy (or smooth) gradation in the appearanceof the test sample of Comparative Example 3. On the other hand,referring to the graph of FIG. 6, the value of b* shows a moderatelyrising tendency even at the center portion of each layer. No step-likechanges in the value of b* may be shown at the interlayer boundary suchthat it is difficult to discern where the boundary is located. Inparticular, the boundary between the first and second layers and thatbetween the third and fourth layers shows linear transition. From thisit is seen that the appearance of the sintered body sample of thepresent invention presents smooth gradation. This result is thought tobe ascribable to the fact that applying the vibration at the time ofcharging of the first to fourth powders causes the powders to be mixedbetween the adjacent layers in a region of the boundary between theupper and lower layer, and thus makes it smaller a difference in thepigment ratio between the adjacent layers. By the way, the value of b*is changed more acutely between the second and third layers than atother portions of the graph. This is presumably due to the markeddifference in the contents of the pigment(s) between the second andthird powders.

TABLE 14 Erbium Iron oxide/ oxide/ Example 4 mass % mass % Low addition0.15 0.05 ratio powder High addition 0.5 0.1 ratio powder

TABLE 15 First Second Third Fourth Example 1 powder powder powder powderLow addition  0% 20% 80% 100% ratio powder High addition 100% 80% 20% 0% ratio powder

Examples 6 to 15

From a composition fabricated by laminating zirconia powders ofdifferent pigment compositions, sintered body samples, which are adaptedto serve as a dental prosthesis, were fabricated. The chromaticity ofthe sintered body sample of each powder that forms each layer wasmeasured. The flexural strength, fracture toughness and the peak ratioof the monoclinic crystal following the hydrothermal treatment regardingthe sintered body of Example 9 were also measured.

Initially, primary powder was prepared in the same way as in Examples 1to 4. The primary powder was divided into four sets of powders, that is,first to fourth powders. In Examples 6 to 15, pigments shown in thefollowing Tables 6 to 16 were added to the powders. The numericalvalues, shown in the Tables, represent values of the addition rates ofthe pigments to the powder mixture of zirconia and yttria (100 mass %).Secondary powders of the first to fourth powders were prepared in thesame way as in Examples 1 to 4 except that the low addition ratio powderand the high addition ratio powder were not fabricated and thatdifferent values of the pigments are used.

Then, a shaped body sample was prepared in the same way as in Examples 1to 4. The shaped body was then fired at 1000° C. for two hours to form apre-sintered body sample. The pre-sintered body sample was then shapedinto a crown shape, using the CAD/CAM system (Katana system, KurarayNoritake Dental Inc.). The pre-sintered body sample was then fired at1500° C. for two hours to form a sintered body sample. The length of thesintered body sample of the first to fourth powders along the layeringdirection was 8 mm.

In each of the sintered body samples of Examples 6 to 16, colorgradation changing from pale yellow to yellow-white color was noticedfrom the local region corresponding to the first layer towards the localregion corresponding to the fourth layer of the composition, thuspresenting the appearance similar to that of a natural tooth.

TABLE 16 Erbium Iron Praseodymium Chromium oxide/ oxide/ oxide/ oxide/Example 6 mass % mass % mass % mass % Fourth powder 0 0 0 0 Third powder0.02 0.04 0 0.0002 Second powder 0.08 0.16 0 0.0008 First powder 0.100.20 0 0.0010

TABLE 17 Erbium Iron Praseodymium Chromium oxide/ oxide/ oxide/ oxide/Example 7 mass % mass % mass % mass % Fourth powder 0 0 0 0 Third powder0.1 0.02 0 0.0002 Second powder 0.4 0.08 0 0.0008 First powder 0.5 0.100 0.0010

TABLE 18 Erbium Iron Praseodymium Chromium oxide/ oxide/ oxide/ oxide/Example 8 mass % mass % mass % mass % Fourth powder 0.10 0 0 0 Thirdpowder 0.15 0.026 0 0.0002 Second powder 0.30 0.104 0 0.0008 Firstpowder 0.35 0.130 0 0.0010

TABLE 19 Erbium Iron Praseodymium Chromium oxide/ oxide/ oxide/ oxideExample 9 mass % mass % mass % /mass % Fourth powder 0.15 0.05 0 0 Thirdpowder 0.22 0.06 0 0 Second powder 0.43 0.09 0 0 First powder 0.50 0.100 0

TABLE 20 Erbium Iron Praseodymium Chromium oxide/ oxide/ oxide/ oxide/Example 10 mass % mass % mass % mass % Fourth powder 0.15 0.050 0 0Third powder 0.19 0.066 0 0 Second powder 0.31 0.114 0 0 First powder0.35 0.130 0 0

TABLE 21 Erbium Iron Praseodymium Chromium oxide/ oxide/ oxide/ oxide/Example 11 mass % mass % mass % mass % Fourth powder 0 0 0 0 Thirdpowder 0.07 0.014 0 0 Second powder 0.28 0.056 0 0 First powder 0.350.070 0 0

TABLE 22 Erbium Iron Praseodymium Chromium oxide/ oxide/ oxide/ oxide/Example 12 mass % mass % mass % mass % Fourth powder 0.10 0.005 0.005 0Third powder 0.48 0.024 0.024 0 Second powder 1.62 0.081 0.081 0 Firstpowder 2.00 0.100 0.100 0

TABLE 23 Erbium Iron Praseodymium Chromium oxide/ oxide/ oxide/ oxide/Example 13 mass % mass % mass % mass % Fourth powder 0.10 0.005 0 0Third powder 0.15 0.030 0.001 0 Second powder 0.30 0.105 0.004 0 Firstpowder 0.35 0.130 0.005 0

TABLE 24 Erbium Iron Praseodymium Chromium oxide/ oxide/ oxide/ oxide/Example 14 mass % mass % mass % mass % Fourth powder 0 0 0 0 Thirdpowder 0.02 0.001 0.001 0 Second powder 0.08 0.004 0.004 0 First powder0.10 0.005 0.005 0

TABLE 25 Erbium Iron Praseodymium Chromium oxide/ oxide/ oxide/ oxide/Example 15 mass % mass % mass % mass % Fourth powder 0.5 0.025 0.025 0Third powder 0.8 0.040 0.040 0 Second powder 1.7 0.085 0.085 0 Firstpowder 2.0 0.100 0.100 0

The chromaticity and the color difference of the sintered body samplesof the first to fourth powders were measured in the same way as inExamples 1 to 4. Tables 26 to 35 show the values of chromaticity. Tables36, 37 show the values of the color difference.

It is thought that chromaticity of each powder represents chromaticityof each point of the zirconia sintered body sample fabricated fromlayered body samples of a plurality of powders. The combination of thefour sintered body samples of Example 9 presents bright color on thewhole. On the other hand, the combination of the four sintered bodysamples of Example 10 presents dark color on the whole.

In the sintered body sample of the first layer, L* was 58 to 76, a* was−2 to 8 and b* was 5 to 27. In the sintered body sample of the secondlayer, L* was 66 to 81, a* −2 to 6 and b* 4 to 21. In the sintered bodysample of the third layer, L* was 69 to 83, a* −2 to 2 and b* 3 to 17.In the sintered body sample of the fourth layer, L* was 71 to 84, a* −2to 1 and b* 2 to 15.

The color difference between the sintered body sample of the first layerand that of the second layer was 3 to 15. The color difference betweenthe sintered body sample of the second layer and that of the third layerwas 1 to 11. The color difference between the sintered body sample ofthe third layer and that of the fourth layer was 1 to 4. The colordifference between neighboring layers showed a decreasing tendency fromthe first layer towards the fourth layer. The color difference betweenthe sintered body sample of the first layer and that of the fourth layerwas 8 to 29. A value obtained by deducting the color difference betweenthe sintered body sample of the first layer and that of the fourth layerfrom the sum of the color difference between the sintered body sample ofthe first layer and that of the second layer, the color differencebetween the sintered body sample of the second layer and that of thethird layer and the color difference between the sintered body sample ofthe third layer and the fourth layer was not larger than unity.

TABLE 26 Example 6 L* a* b* Fourth powder 71.97 0.60 2.10 Third powder70.36 0.61 4.44 Second powder 68.77 0.82 11.22 First powder 64.79 0.9319.76

TABLE 27 Example 7 L* a* b* Fourth powder 74.33 −0.75 5.24 Third powder73.72 −0.63 6.35 Second powder 73.11 1.70 9.59 First powder 71.59 2.9213.65

TABLE 28 Example 8 L* a* b* Fourth powder 71.97 0.60 2.10 Third powder71.38 0.64 3.68 Second powder 70.80 1.37 8.27 First powder 69.35 1.7614.04

TABLE 29 Example 9 L* a* b* Fourth powder 73.79 −0.90 6.64 Third powder73.30 −0.78 7.57 Second powder 72.81 1.65 10.26 First powder 71.59 2.9213.65

TABLE 30 Example 10 L* a* b* Fourth powder 73.79 −0.90 6.64 Third powder72.80 −0.82 7.62 Second powder 71.81 0.88 10.46 First powder 69.35 1.7614.04

TABLE 31 Example 11 L* a* b* Fourth powder 83.97 −1.89 4.17 Third powder81.18 −1.78 5.04 Second powder 78.42 0.37 7.57 First powder 71.52 1.4910.75

TABLE 32 Example 12 L* a* b* Fourth powder 75.77 −1.41 5.70 Third powder71.82 −1.13 8.40 Second powder 67.92 4.47 16.24 First powder 58.16 7.4026.1

TABLE 33 Example 13 L* a* b* Fourth powder 75.77 −1.41 5.70 Third powder74.33 −1.31 6.80 Second powder 72.91 0.71 10.01 First powder 69.35 1.7614.04

TABLE 34 Example 14 L* a* b* Fourth powder 83.97 −1.89 4.17 Third powder82.13 −1.87 4.37 Second powder 80.31 −1.57 4.96 First powder 75.77 −1.415.70

TABLE 35 Example 15 L* a* b* Fourth powder 72.49 0.97 14.5 Third powder69.28 1.18 16.03 Second powder 66.10 5.26 20.49 First powder 58.16 7.4026.10

TABLE 36 Examples 6 7 8 9 10 Color difference between third 2.8 1.3 1.71.1 1.4 and fourth powders ΔE*ab1 Color difference between 7.0 4.0 4.73.7 3.5 second and third powders ΔE*ab2 Color difference between first9.4 4.5 6.0 3.8 4.4 and second powders ΔE*ab3 Color difference betweenfirst 19.1 9.6 12.3 8.3 9.0 and fourth powders ΔE*ab4 (ΔE*ab1 + ΔE*ab2 +ΔE*ab3) − 0.1 0.2 0.1 0.3 0.3 ΔE*ab4

TABLE 37 Examples 11 12 13 14 15 Color difference between third 2.9 4.81.8 1.9 3.6 and fourth powders ΔE*ab1 Color difference between 4.3 10.44.0 1.9 6.8 second and third powders ΔE*ab2 Color difference betweenfirst 7.7 14.2 5.5 4.6 10.0 and second powders ΔE*ab3 Color differencebetween first 14.5 28.4 11.0 8.4 19.5 and fourth powders ΔE*ab4(ΔE*ab1 + ΔE*ab2 + ΔE*ab3) − 0.4 1.0 0.3 0 0.9 ΔE*ab4

A zirconia sintered body sample was independently prepared from each ofthe first to fourth powders of Example 9 and the flexural strength,fracture toughness and the peak ratio of the monoclinic crystalfollowing the hydrothermal treatment were measured. The results ofmeasurement are shown in Table 38. The flexural strength of the zirconiasintered body sample was measured pursuant to JISR1601. The fracturetoughness of the zirconia sintered body sample was measured pursuant toJISR1607. The hydrothermal treatment test was conducted pursuant toISO13356 under a condition of at 180° C., 1 MPa for five hours. Afterthe hydrothermal treatment test, the X-ray diffraction pattern of thezirconia sintered body sample was measured, using CuKα rays, to measurethe peak ratio of the monoclinic crystal, that is, the extent of phasetransition to the monoclinic crystal caused by the hydrothermaltreatment test. In any of the sintered body samples, the flexuralstrength was not less than 1200 MPa, the fracture toughness was not lessthan 4 MPa·m^(1/2) and the peak ratio of the monoclinic crystal was notlarger than unity. It is thought that, since the zirconia sintered bodysamples of the other Examples are similar in composition, similarresults would be obtained with these Examples. Test results of theflexural strength and the fracture toughness were similar to thoseobtained with the load applied to the boundary of the laminated bodysamples.

As for the second powders, the flexural strength of the pre-sinteredbody sample, prepared by firing at 1000° C. for two hours, was alsomeasured pursuant to JISR1601. The flexural strength of the sinteredbody sample of the second powder was 41 MPa. This value was similar tothat obtained on testing under a load applied to the boundary of thelaminated body samples.

TABLE 38 Flexural Fracture Peak ratio * strength/ toughness/ ofmonoclinic Samples for measurement MPa MPa · √ m crystal Sintered bodyof first 1210 4.3 0.58 powders Sintered body of second 1216 4.3 0.59powders Sintered body of third 1204 4.3 0.60 powders Sintered body offourth 1202 4.3 0.59 powders

Example 16

In the above Examples, the content of yttria was 4 mol % in terms of thetotal mols of zirconia and yttria. In Example 16, a sintered body samplewith the yttria content of 3 mol % was prepared to measure thechromaticity. Except the yttria content, the sintered body sample usedfor measurement was the same as that of Example 4 shown in Tables 14 and15. Table 39 shows measured results. Comparison with the chromaticityshown in Table 12 indicates that, if the yttria content is lowered, L*tends to decrease, while a* and b* tend to increase.

TABLE 39 Example 16 L* a* b* Sintered body of fourth 73.20 −1.35 5.72powders Sintered body of third 69.78 0.42 9.84 powders Sintered body ofsecond 59.52 5.75 22.20 powders Sintered body of first 56.10 7.52 26.32powders

The zirconia sintered body as well as the composition and thepre-sintered body for the zirconia sintered body has been explained inthe above exemplary embodiments. It should be noted however that thepresent invention is not limited to the above described exemplaryembodiments and a variety of modifications, changes and improvements maybe made of the elements herein disclosed, inclusive of elements ofclaims, exemplary embodiments and Examples as well as drawings, withinthe scope of the invention, based on the fundamental technical conceptof the present invention. It is also possible to make a diversity ofcombinations, substitutions and selections of elements herein disclosed,inclusive of elements of claims, exemplary embodiments and Examples aswell as drawings, within the scope of the invention.

Further problems, objects and development embodiments of the presentinvention will become apparent from the entire disclosures inclusive ofthe claims.

It should be understood that, as regards the range of numerical values,any arbitrary numerical values or sub-ranges contained in the ranges ofnumerical values set out herein ought to be construed that they areexplicitly stated even in the absence of such explicit statements in thepresent description.

Part or all of the above described exemplary embodiments may also bestated as in supplementary notes shown below, though not restrictively.

[Supplementary Note 1]

A zirconia sintered body, which has

a flexural strength of not less than 1100 MPa measured on a test sampleof the zirconia sintered body pursuant to JISR1601;the test sample being formed by:preparing a plurality of zirconia powders, each containing zirconia anda stabilizer(s) that suppresses phase transition of zirconia, theplurality of zirconia powders differing in composition;laminating the plurality of the zirconia powders to form a zirconialamination composition; andsintering the zirconia lamination composition to form a zirconiasintered body;the flexural strength being measured under a condition that a load pointof a three-point bending test is positioned at a position of aninterlayer boundary of the plurality of zirconia powders, the interlayerboundary traversing the test sample of the sintered body along adirection of load application.

[Supplementary Note 2]

The zirconia sintered body according to supplementary note, wherein;

the flexural strength is not less than 1200 MPa.

[Supplementary Note 3]

The zirconia sintered body according to supplementary note, wherein;

when a zirconia pre-sintered body is prepared by pre-sintering thezirconia composition at 800° C. to 1200° C.,a flexural strength of a test sample of the pre-sintered body, measuredpursuant to JISR1601, is not less than 90% of a flexural strength of thezirconia pre-sintered body obtained by pre-sintering one composition ofthe plurality of zirconia powders alone at the same temperature as apre-sintering temperature of the test sample,the flexural strength being measured under the condition that a loadpoint of the three-point bending test is positioned at the position ofthe interlayer boundary of the plurality of zirconia powders, theinterlayer boundary traversing the test sample of the sintered bodyalong the direction of load application.

[Supplementary Note 4]

The zirconia sintered body according to supplementary note, wherein, theplurality of zirconia powders contain a pigment(s) in different ratios,respectively.

[Supplementary Note 5]

The zirconia sintered body according to supplementary note, wherein, theflexural strength, as measured pursuant to JISR1601, of each sinteredbody, obtained by sintering one composition of the plurality of zirconiapowders alone at 1500° C., is not less than 1100 MPa.

[Supplementary Note 6]

A zirconia pre-sintered body, which has

a flexural strength of a test sample of the zirconia pre-sintered body,measured pursuant to JISR1601, which is not less than 90% of theflexural strength of a comparative zirconia pre-sintered body;the test sample being formed by:preparing a plurality of zirconia powders, each containing zirconia anda stabilizer that suppresses phase transition of zirconia, the pluralityof zirconia powders differing in composition;laminating the plurality of zirconia powders to form a zirconialamination composition; andpre-sintering the zirconia lamination composition at 800° C. to 1200° C.to form a zirconia pre-sintered body;the comparative zirconia pre-sintered body being formed by pre-sinteringone composition of the plurality of zirconia powders alone at the sametemperature as a pre-sintering temperature of the test sample;the flexural strength being measured under a condition that a load pointof a three-point bending test is positioned at a position of aninterlayer boundary of the zirconia powders, the interlayer boundarytraversing the test sample of the pre-sintered body along a direction ofload application.

[Supplementary Note 7]

The zirconia sintered body according to supplementary note, wherein, theplurality of zirconia powders contain a pigment(s) in different ratios,respectively.

[Supplementary Note 8]

A zirconia pre-sintered body, wherein,

when a test sample is placed on a ground with one of two bottom surfacesthat has been deformed to a concave shape directed downwards; (a maximumgap between a deformed concave bottom surface and a groundsurface)/(distance between portions of the test sample contacting theground surface along a widthwise direction)×100 is 0.15 or less,the test sample being formed by:preparing a plurality of zirconia powders, each containing zirconia anda stabilizer(s) that suppresses phase transition of zirconia, theplurality of zirconia powders differing in composition;laminating the plurality of zirconia powders to form a zirconiacomposition;firing the zirconia composition at 800° C. to 1200° C. to form azirconia pre-sintered body; the pre-sintered body being shaped to a formof a rectangular parallelepiped 50 mm in width[length], 10 mm in heightand 5 mm in depth[thickness] as the test sample, and two surfaces of thetest sample of 50 mm in width and 5 mm in depth are taken to be thebottom surfaces; boundary surfaces formed by laminating of the pluralityof zirconia powders extending in the same direction as the bottomsurfaces; andsintering the pre-sintered body at 800° C. to 1200° C. for two hours.[Supplementary Note 9] The zirconia pre-sintered body according to thesupplementary note, wherein (the maximum gap between the deformedconcave bottom surface and the ground surface)/(distance between theportions of the test sample contacting the ground surface along thewidthwise direction)×100 is 0.1 or less.

[Supplementary Note 10]

The zirconia pre-sintered body according to the supplementary note,wherein the plurality of zirconia powders contain a pigment(s) inrespective different ratios.

[Supplementary Note 11]

The zirconia sintered body according to the supplementary note, wherein,

the zirconia sintered body is prepared by a method comprising: sinteringthe zirconia pre-sintered body at 1400° C. to 1600° C.

INDUSTRIAL APPLICABILITY

The zirconia sintered body according to the present invention may be putto a variety of uses, including dental materials, such as prostheses,connection parts for optical fibers, such as ferrules and sleeves, avariety of tools, such as crushing balls and grinding tools, a varietyof components, such as screws, bolts and nuts, a variety of sensors,electronic parts, and ornaments, such as watch bands. In using thezirconia sintered body for a dental material, it may be used as, forexample, coping, a framework, a crown, a crown bridge, an abutment, animplant, an implant screw, an implant fixture, an implant bridge, animplant bar, a bracket, a dental plate, inlay, unlay, onlay, a wire forcorrection or a laminate veneer.

REFERENCE SIGNS LIST

-   10 zirconia sintered body-   20, 22 pre-sintered body-   20 a, 22 a bottom surface-   21 a, 22 a first and second surfaces-   23 a to 23 d first to fourth layers-   30 ground surface-   A to D first to fourth points-   P one end-   Q opposite end-   X first direction-   Y second direction

1. A method for preparing a zirconia composition, comprising: preparinga plurality of powders for lamination adapted to form a laminationcomprising layers of the plurality of powders for lamination; andlaminating layers of the plurality of powders for lamination in a mold;wherein: the plurality of powders for lamination comprise: zirconia, astabilizer suppressing phase transition of zirconia, and a pigment atdifferent pigment content ratios; during the laminating, the mold isvibrated after at least two layers of the of the plurality powders forlamination are charged into the mold.
 2. The method according to claim1, wherein during the laminating, the mold is vibrated each time a layerof the plurality of powders for lamination is charged into the mold. 3.A method for preparing a zirconia composition, comprising: preparing alow addition ratio powder and a high addition ratio powder; mixing thelow addition ratio powder and the high addition ratio powder to form atleast one powder for lamination; and laminating at least two powdersselected from the group consisting of the low addition ratio powder, thehigh addition ratio powder and the at least one powder for laminationinto the mold; wherein: each of the low addition ratio powder and thehigh addition ration powder comprises: zirconia, a stabilizersuppressing phase transition of zirconia, and a pigment; and the lowaddition ratio powder and the high addition ratio powder differ fromeach other in pigment content ratios.
 4. The method according to claim3, wherein during the laminating, the mold is vibrated after chargingthe at least two powders into the mold.
 5. The method according to claim3, wherein: during the mixing, two or more powders for lamination withdifferent mixing ratios of the low addition ratio powder and the highaddition ratio powder are prepared; and during the laminating, thepowders are laminated on one another so that the contents of the lowaddition ratio powder and the high addition ratio powder are varied inorder.
 6. The method according to claim 3, wherein during thelaminating, after charging one powder into the mold, an upper surface ofthe powder is made flat.
 7. The method according to claim 3, whereinduring the laminating, the powders are laminated so that the pigmentcontents in the powders are varied in order.
 8. A method for preparing azirconia pre-sintered body, comprising: preparing a zirconia compositionaccording to the method according to claim 3; and firing the zirconiacomposition at 800° C. to 1200° C.
 9. A method for preparing a zirconiasintered body, comprising: preparing a zirconia composition according tothe method according to claim 3; and firing the zirconia composition at1400° C. to 1600° C.
 10. A method for preparing a zirconia sinteredbody, comprising: preparing the zirconia pre-sintered body according tothe method according to claim 8; and firing the pre-sintered body at1400° C. to 1600° C.
 11. A zirconia sintered body, manufactured by themethod according to claim
 9. 12. The zirconia sintered body according toclaim 11, wherein: when the zirconia sintered body is prepared bysintering the zirconia composition, a flexural strength of the zirconiasintered body pursuant to JISR1601 is not less than 1100 MPa as measuredwith a load point of a three-point bending test aligned with aninterlayer boundary of the laminating of the zirconia powders; and theinterlayer boundary traverses a test sample of the zirconia sinteredbody along a direction of load application.
 13. The zirconia sinteredbody according to claim 12, wherein the flexural strength is not lessthan 1200 MPa.
 14. The zirconia sintered body according to claim 11,wherein; when a zirconia pre-sintered body is prepared by pre-sinteringthe zirconia composition at 800° C. to 1200° C., the flexural strengthof the pre-sintered body, as measured with a load point of a three-pointbending test aligned with the interlayer boundary pursuant to JISR1601,is not less than 90% of the flexural strength of the zirconiapre-sintered body obtained on pre-sintering one composition of thezirconia powders alone at the same temperature as a pre-sinteringtemperature of the test sample; and the boundary traverses a test sampleof the pre-sintered body along a direction of load application.
 15. Thezirconia sintered body according to claim 11, wherein when thecomposition is pre-sintered at 800° C. to 1200° C. to form a zirconiapre-sintered body: the pre-sintered body is shaped to a form of arectangular parallelepiped 50 mm in width or length, 10 mm in height and5 mm in depth or thickness as a test sample; two surfaces of the testsample of 50 mm in width and 5 mm in depth are taken to be bottomsurfaces; interlayer boundary surfaces formed by laminating of thezirconia powders then extending in the same direction as the bottomsurfaces; the test sample is fired at 1500° C. for two hours; and thetest sample is placed on a ground with one of the two bottom surfacesthat has been deformed to a concave shape directed downwards; wherein aratio of (a maximum gap between the deformed concave bottom surface anda ground surface)/(distance between portions of the test samplecontacting the ground surface along a widthwise direction), multipliedby 100, is 0.15 or less.
 16. The zirconia sintered body according toclaim 11, wherein, provided that, on a straight line extending from oneend to an opposite end in a first direction: a chromaticity (L*, a*, b*)in an L*a*b* color chromaticity diagram of a first point in a domainfrom the one end to a point of 25% of a total length is (L1, a1, b1);and a chromaticity (L*, a*, b*) in the L*a*b* color chromaticity diagramof a second point in a domain from an opposite end to a point of 25% ofthe total length is (L2, a2, b2); L1 is not less than 58.0 and notlarger than 76.0; a1 is not less than −1.6 and not larger than 7.6; b1is not less than 5.5 and not larger than 26.7; L2 is not less than 71.8and not larger than 84.2; a2 is not less than −2.1 and not larger than1.8; b2 is not less than 1.9 and not larger than 16.0; L1<L2; a1>a2; andb1>b2; wherein there is no change in increasing or decreasing tendencyof chromaticity in the L*a*b* color chromaticity diagram from the firstpoint towards the second point. 17-30. (canceled)
 31. A zirconiapre-sintered body, prepared by the method according to claim
 8. 32. Thezirconia pre-sintered body according to claim 31, wherein: a flexuralstrength of a test sample of the zirconia pre-sintered body, measuredpursuant to JISR1601, is not less than 90% of the flexural strength of acomparative zirconia pre-sintered body; the comparative zirconiapre-sintered body being formed by pre-sintering one of the zirconiapowders alone at the same temperature as a pre-sintering temperature ofthe test sample; and the flexural strength being measured under acondition that a load point of three-point bending test is positioned ata position of an interlayer boundary of the zirconia powders, theinterlayer boundary traversing the test sample of the sintered bodyalong a direction of load application.
 33. The zirconia sintered bodyaccording to any one of claim 31, wherein, when the pre-sintered body isshaped to a form of a rectangular parallelepiped 50 mm in width orlength, 10 mm in height and 5 mm in depth or thickness as a test sample:two surfaces of the test sample of 50 mm in width and 5 mm in depth, aretaken to be bottom surfaces; interlayer boundary surfaces formed bylaminating of the zirconia powders extending in the same direction asthe bottom surfaces; the test sample is fired at 1500° C. for two hours;and the test sample is placed on a ground with one of the two bottomsurfaces that has been deformed to a concave shape directed downwards;wherein a ratio of (a maximum gap between a deformed concave bottomsurface and a ground surface)/(distance between portions of the testsample contacting the ground surface along a widthwise direction),multiplied by 100, is 0.15 or less.
 34. A zirconia composition preparedby the method according to claim
 3. 35-36. (canceled)